There are a number of acoustical phenomena that are rarely noticed consciously by the average listener in a typical environment but nevertheless detract from optimal audio quality. One is acoustical crosstalk, which occurs when two loudspeakers play the same signal, creating a phantom center image. It is well known that acoustical crosstalk produces comb filtering with deep spectral notches, resulting in undesirable coloration and a loss of spectral information.
When two loudspeakers play the same signal, the resulting phantom center image differs from one produced by a real center speaker. In particular and as noted, acoustical crosstalk produces a comb-filtering effect, with cancellations that are typically in the frequency range needed for the intelligibility of speech. In addition, the phantom image is not as stable as that of a real center speaker, because it tends to follow the listener toward the nearest speaker due to the precedence effect. There are additional problems relating to mono-compatibility and speaker/headphone compatibility.
One solution to problems of phantom center images is simply to add a real center speaker. This approach had the advantage of providing a stable center image. However, for reasons of cost and space, many consumer audio and television systems do not include a center speaker. Therefore, an approach that works over two speakers is desired.
Another solution to the problem of acoustic crosstalk is to cancel it before it happens, using various crosstalk cancellation techniques. However, at mid and high frequencies, this is effective only within a relatively small “sweet spot,” which limits the usefulness of this technique for typical television viewing and other situations involving multiple listeners in arbitrary positions.
Another way to address the non-flat magnitude response caused by acoustical crosstalk is to apply inverse filters to the left and right signals. However, the frequencies of the comb filter notches vary greatly depending on the relative positions of the speakers and listener. For example, the cancellation frequencies increase as the angle subtended by the speakers becomes narrower, such as when the listener moves further back. In addition, as the listener moves to the side and is no longer equidistant from the speakers, the notches move closer together and become different for each ear. Without a good estimate of the relative positions, it would be impossible to accurately equalize the effects of the crosstalk.